Boise Police say the cyclist, a man in his 30s, was riding north on Federal Way at the intersection with Gowen Road just before 5 p.m.
Police say the truck was traveling eastbound on Gowen Road and hit the cyclist.
Officers say the cyclist appeared to have suffered head and leg injuries, but was conscious and alert on the scene. He was not wearing a helmet at the time of the crash.
Later Tuesday evening, police updated the injuries to the cyclist in the hospital as serious and potentially life-threatening.
Police say that it appears the driver of the truck had a green light at the intersection and no citations have been issued at this time.
The crash is under investigation.